The invention is related to a pressure gauge device that is operated as a thermal conduction manometer and methods of measuring pressure with a pressure gauge device of this kind.
The specific thermal conductivity of a gas at low pressures, when the mean free path length is comparable with the magnitude of the vessel dimensions, is dependent on gas pressure, which is made use of in the thermal conduction manometer for measuring pressure. In the simplest case a thermal conduction manometer comprises a self-supporting resistance wire in the gas to be measured. The resistance wire is heated with constant electrical power. As soon as the mean free path length of the gas particles reaches the dimension of the wire diameter, the thermal conductivity of the gas becomes dependent on pressure. With decreasing pressure the heat current conducted by the gas drops, so that the wire temperature is dependent on pressure at constant heating power. Below a lower pressure limit the heat discharge through the gas can be ignored compared to heat discharge through the leads or through thermal radiation, so that the conducted heat current is independent of pressure and the temperature of the resistance wire is thus constant.
The wire temperature can be determined from the resistance of the wire. In the Pirani manometer the resistance is measured by a Wheatstone bridge circuit, in which one resistor is formed by the measuring resistance wire and a reference resistor, of the same design as the test resistance wire, is operated under constant pressure.
Thermal conduction manometers designed as Pirani manometers generally exhibit the following disadvantages. The resistance wires, with as small a diameter as possible to produce as high an initial pressure as possible (see above), usually consist of a material of relatively high resistance (e.g. tungsten or molybdenum). High-resistance materials will only allow operation of the measuring bridge circuit with direct current. Furthermore, the susceptibility to electromagnetic interference increases, which is a particular disadvantage in plasma physics experiments. Consequently the accuracy of familiar Pirani manometers is limited. Additionally, manometer design with self-supporting resistance wires is sensitive to mechanical stress and recipient ventilation. Finally, no compact design of the familiar Pirani manometer has yet been implemented, meaning that use of these manometers is generally restricted to unspecific pressure measurements in the operation of vacuum plants.
Other thermal conduction manometers, known from U.S. Pat. No. 5,557,972, DE-OS 43 10 324 and DE-OS 44 13 349 for example, contain a measuring resistor that, instead of the above mentioned wire form, is of a flat layer form. These resistance manometers may exhibit higher sensitivity but have a drawback in terms of dynamic response and mechanical stability. To achieve high sensitivity there are namely measures for thermal isolation of the measuring resistor from the remainder of the manometer. This results in relatively long times for setting a thermal equilibrium between the gas to be measured and the measuring resistor, so changes in pressure cannot be detected that are shorter than the time for setting the thermal equilibrium. In conventional thermal conduction manometers the measuring resistor is created on membranes produced in semiconductor technology (thickness of the measuring resistor and the carrier membrane approx. 1 xcexcm), which means extreme mechanical sensitivity. Consequently the designs of conventional thermal conduction manometers with a layer shaped measuring resistor are limited to miniaturized sensors. This in turn leads to reduced sensitivity when measuring pressure.
DE-OS 43 08 434 describes an electronic circuit for temperature compensation in a regulated thermal conduction manometer.
The object of the invention is to provide an improved pressure gauge device based on the principle of thermal conduction that exhibits an extended range of use, is designed to be especially compact and unsusceptible to interference, and can be operated with high accuracy.
This purpose is resolved by a pressure gauge device with the features of patent claim 1. Advantageous implementations of the invention result from the dependent claims.
The invention is based on the idea of using a layer structure, in contrast to conventional structures, in a pressure gauge device based on the principle of thermal conduction, and in which at least one measuring resistor and a thermal conducting layer are arranged in layer form on a carrier foil.
The thermal conducting layer is connected to a thermal contact layer acting as a heat bath (high thermal capacity) and therefore serves for predetermined thermal coupling of the measuring resistor with the remainder of the pressure gauge device. Unlike conventional thermal conduction manometers, the invention thus creates in the thermal conducting layer a means of providing defined heat discharge from the measuring resistor. The thermal conducting layer can also be part of the carrier foil if the latter exhibits sufficient thermal conduction comparable to that of gold.
The invented design allows, for the first time, acceleration of the setting of thermal equilibrium of the measuring resistor, speeding up of the response time and thus the possibility of detecting extremely fast pressure changes (of the order of kHz).
A further advantage of attaching a thermal conducting layer is the possibility of feedback operation of the pressure gauge device. Feedback operation, details of which are explained below with reference to FIG. 2, means that the measuring resistor is altered in a defined way by direct or indirect heating, e.g. is set to a constant temperature. Seeing as the necessary heating power depends on the heat discharge and thus the pressure of the medium, the momentary pressure value can be determined direct from the heating power. The temperature equilibrium of the measuring resistor, subject to the effect of a heat source (possibly an extra heating resistor) and the thermal conducting layer acting as a heat sink, can be set at high speed in the required temperature range.
The thermal conducting layer, possibly by attaching an absorber layer and/or surface structuring, and the measuring resistors or carrier foil are dimensioned so that the heat current from the measuring resistor to the medium passes through the layers for the most part vertically. Addition of a thermal conducting layer allows defined setting of a temperature gradient between the measuring resistor and the medium to be measured. In a preferred implementation of the invention, the measuring resistor is provided on one side of the carrier foil and the thermal conducting layer on the opposite side in a region corresponding to the measuring resistor. Other layer sequences are also possible however, allowing essentially vertical heat current from the measuring resistor through the thermal conducting layer to the medium so that the sensitivity or response time of the pressure gauge device is predetermined. On the opposite side to the carrier foil the measuring resistor can be in direct contact with the medium or bear an absorber layer (single- or double-layer).
The pressure gauge device according to the invention is preferably operated as a Pirani manometer with a bridge circuit. In a first embodiment the measuring resistor and a reference resistor of the same design are spaced apart on the carrier foil, the latter being arranged in a block of good thermal conduction that has a recess accomodate the medium to be measured in the region in which the measuring resistor is arranged on the carrier foil, and form-fits around the region of the carrier foil where the reference resistor is arranged. Thus the reference resistor is entirely connected to a heat bath (infinitely high thermal capacity) so that it is not subjected to any pressure-dependent changes of temperature.
In a second embodiment of the invention the design that has been explained is modified so that the reference resistor like the measuring resistor is arranged on a self-supporting region of the carrier foil. In this case the reference resistor is not enclosed by a heat bath but brought into thermal contact with the medium to be measured. The pressure gauge device contains a sensor part with a layer structure of carrier foil, measuring resistor and thermal conducting layer, as described above, and a reference part likewise with a layer structure of reference resistor, carrier foil and thermal conducting layer. The sensor and reference parts will preferably have the same geometry, the thickness of their thermal conducting layers being different however. In what follows therefore, a distinction is made between a sensor thermal conducting layer and a reference thermal conducting layer in connection with the second embodiment. The reference thermal conducting layer is preferably thicker (by a factor of about 4 to 5 for example) than the sensor thermal conducting layer. In a preferred implementation the measuring resistor can have a separate heating resistor (sensor heating resistor) as can the reference resistor (reference heating resistor).
The pressure gauge device according to the invention allows manometer design with enhanced mechanical stability. This is a result of the measures for improving thermal coupling of the measurement or reference resistor with the environment. Consequently a new design with stable carrier films can be produced that are sufficiently rugged in ventilating operations and allow a larger sensor area compared to conventional resistance manometers with a layer structure. This in turn increases the sensitivity of the pressure gauge device.
The layer structure of the invention also allows miniaturization of the pressure-sensitive sensor part of the pressure gauge device. In a further embodiment of the invention a large number of pressure gauge devices are therefore configured like a matrix in a thermal conduction manometer for position-sensitive resolution in measurement of pressure.
In an advantageous use of the invented device, pressure profiles are detected in a medium with a large number of pressure sensors arranged linear or flat, in the region of interest, of a recipient for example, and intended for simultaneous detection of the pressure values of all pressure sensors.